<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <input id="a" type="text">
    <select>
        <option selected="selected" value="default">请选择</option>
        <option value="add">+</option>
        <option value="subtract">-</option>
        <option value="multiply">*</option>
        <option value="divide">/</option>
    </select>
    <input id="b" type="text">
    <input id ="btn" type="button" value="=">
    <input id="c" type="text">

    <script src="js/jquery-1.12.4.min.js"></script>
    <script>
        // 获取元素
        var $se = $("select")

        var $a = $("#a")
        var $b = $("#b")
        var $c = $("#c")
        var $btn = $("#btn")
        $btn.click(function () {            
            if($se.val() == "add") {
                $c.val(parseInt($a.val()) + parseInt($b.val()))
            }
            if ($se.val() == "subtract") {
                $c.val(parseInt($a.val()) - parseInt($b.val()))
            }
            if ($se.val() == "multiply") {
                $c.val(parseInt($a.val()) * parseInt($b.val()))
            }
            if ($se.val() == "divide") {
                $c.val(parseInt($a.val()) / parseInt($b.val()))
            }
            if ($se.val() == "default") {
                $c.val("")
            }                
        })
    </script>
</body>
</html>